Obesity hypoventilation syndrome (OHS) occurs when a very obese person does not breathe enough oxygen while sleeping.

The exact cause of OHS in unknown. Most (but not all) patients with the syndrome have a form of sleep apnea.

OHS is believed to result from both a defect in the brain's control over breathing and excessive weight (due to obesity) against the chest wall, which makes it hard for a person to take a deep breath. As a result, the blood has too much carbon dioxide and not enough oxygen. (See respiratory acidosis). Patients with OHS have chronic fatigue due to sleep loss, poor sleep quality, and chronic hypoxia.

Morbid (massive or excessive) obesity is the main risk factor.

The main symptoms of OHS are due to sleep deprivation, and include:

• Excessive daytime sleepiness
• Falling asleep at inappropriate times during the day
• Increased risk for accidents or errors at work
• Depression

Symptoms of chronic hypoxia (low blood oxygen level) can also occur, such as shortness of breath or fatigue after minimal physical effort.

Persons with OHS are usually extremely overweight. Physical signs that suggest OHS include:

• Cyanosis (bluish coloration of the lips, fingers, toes, or skin)
• Signs of right-side heart failure (cor pulmonale ) such as swollen legs or feet, shortness of breath, or feeling fatigued after little exertion.

Tests done to confirm the diagnosis include:

• Sleep study
• Pulmonary function
• Arterial blood gas

The presence of high carbon dioxide (CO₂) levels in the blood when a person is awake helps distinguish OHS from obstructive sleep apnea.

The treatment involves mechanical ventilation to help the person breath. Options include:

• Non-invasive mechanical ventilation (BiPAP or other modes) through a mask that fits tightly over the nose or nose and mouth
• Mechanical ventilation through a tracheostomy (an incision in the neck)

Other therapies are aimed at weight loss, which may lead to reversal of the OHS.

Support groups may help patients with OHS, or their family members, adjust to the lifestyle changes that are required for successful treatment. In addition, support groups may provide information about new treatments that are developed.

When treated, OHS has a good prognosis. Untreated, it can lead to serious heart and blood vessel complications, severe disability or death.

Complications of OHS include complications of excessive sleepiness, such as:

• Increased risk for accidents
• Depression, agitation, irritability
• Sexual dysfunction

OHS may also include cardiovascular complications, such as

• Hypertension
• Cor pulmonale (right heart failure)

Call your health care provider if you experience excessive daytime sleepiness, or any other symptoms that suggest OHS.

Maintain a healthy weight and avoid obesity.

Murray J, Nadel J. Textbook of Respiratory Medicine. 3^rd ed. Philadelphia, Pa: W.B. Saunders Company; 2000:2326-2328.

Moore DP, Jefferson JW. Handbook of Medical Psychiatry. 2nd ed. St. Louis, Mo: Mosby, Inc; 2004:221-223.